JavaScript Functions

A function is a named, reusable block of code. Write it once, call it whenever you need it.

function greet(name) {
  return `Hello, ${name}!`;
}

console.log(greet("Ada"));   // "Hello, Ada!"
console.log(greet("Lin"));   // "Hello, Lin!"

Three parts:

1. Declaration: function greet(name) { … }
2. Parameters: name — values the caller passes in.
3. return: the value the function gives back.

Calling a Function

Use the function’s name followed by () — pass any arguments inside.

function add(a, b) {
  return a + b;
}

console.log(add(2, 3));        // 5
console.log(add(10, 20) + 1);  // 31  (the result is a value)

Returning a Value

return exits the function and hands a value back to the caller. Once return runs, the function is done — code after it doesn’t execute.

function classify(n) {
  if (n > 0) return "positive";
  if (n < 0) return "negative";
  return "zero";
}

console.log(classify(5));   // "positive"
console.log(classify(-3));  // "negative"
console.log(classify(0));   // "zero"

If a function doesn’t return anything, it returns undefined.

function shout(text) {
  console.log(text.toUpperCase());
}

const result = shout("hello");
console.log(result);  // undefined  ← no return

Declaration vs Expression

Two ways to define a function.

Declaration — uses the function keyword as a statement:

function add(a, b) {
  return a + b;
}

Expression — assigns a function to a variable:

const add = function (a, b) {
  return a + b;
};

Both create callable functions. The differences:

• Declarations are hoisted — you can call them before the line where they’re written.
• Expressions aren’t — you have to declare before you call.

// ✅ Declaration: can be called before its definition
console.log(declared());   // "I work"

function declared() {
  return "I work";
}

// ❌ Expression: ReferenceError if called too early
// console.log(expressed());

const expressed = function () {
  return "I work";
};

Anonymous Functions

A function expression doesn’t need a name — you can leave it anonymous and refer to it through the variable.

const square = function (n) {
  return n * n;
};

console.log(square(4));  // 16

You’ll see anonymous functions most often as callbacks — passed to methods like map, filter, setTimeout. Arrow functions (coming up) are usually a nicer fit.

Try It Yourself

// declare double here

console.log(double(7));

function double(n) {
  return n * 2;
}

console.log(double(7));

Up Next

A deeper look at parameters — defaults, rest, destructuring.